High-Homogeniety Solenoids
The magnetic field in a simple solenoid decreases approximately quadratically with the
axial distance from the center of the magnet. The homogeneity at the center of the
magnet can be improved significantly by adding windings to each end of the solenoid
to compensate for this decrease in the field. Mathematically, this technique results
in a very uniform magnetic field at the center of the coil. In practice, however
, winding imperfections limit the homogeneity attained and additional series connected
trim windings are required to achieve the maximum homogeneity. Since the trim coils are
connected in series, this type of magnet has only two current leads and one persistent
switch is used for the complete magnet. The homogeneity of such a magnet is about
± part in 10
5
of the central field in a spherical volume having a diameter that is approximately 1/5
of the bore of the magnet in small magnets.
The limit on this technique is the change in field homogeneity caused by adding or
removing one turn of conductor from the magnet. A typical magnet of this type might
have 3 x 10
4
turns, so each turn contributes 3 x 10
-5
of the total field, to a first approximation. Field homogeneity of 1 part in 10
6
require something other than adding or deleting turns. The procedure used is to
add separately energized trim coils to the magnet so the field contribution of
each coil can be varied. Each of these coils is fitted with a separate persistent
switch. By the nature of this correction, each of these coils operate at currents
lower than the main magnet. Magnets with these separate trim coils have a
homogeneity of about 1 part in 10
6
of the central field. The ratio of the magnet bore diameter to the diameter
of the homogeneous volume typically increases as the specified homogeneity
increases.
Extending this procedure to the next set of trim coils results in coils
operating at still lower currents. These currents are small enough that copper
wires can be used and the coils can be operated at room temperature.
Homogeneities of about 1 part in 10
7
of the central field can be achieved by the addition of these coils.
A great amount of emphasis has been placed on extremely homogeneous and
persistent magnets in recent years. These magnets obviously fulfill a need
in many experiments. Not all NMR experiments require these features, however,
and money can be saved by specifying only the parameters that are required
for your particular experiment.
Magnets of this type are used extensively for NMR experiments as well as
for some precision transport phenomena measurements.
Level Instrumentation